In recent years, the MRAM (Magnetic Random Access Memory) is proposed, and investigations for putting the MRAM to practical use have been proceeded. In the magnetic random access memory, a magnetic body is used for the memory element. Therefore, it operates as a non-volatile random access memory. Further, writing tolerance of more than fifteenth power of ten is guaranteed. Moreover, it can realize switching in a timescale of less than some nanoseconds. Because of these characteristics, it is expected for the magnetic random access memory for the application to a high-speed non-volatile random access memory operated by, especially, more than some 100 MHz.
The magnetic random access memory if formed of a magnetoresistive effect element. The magnetoresistive effect element is formed of a magnetization free layer, an insulating layer, and a magnetization fixed layer, and in general cases, an anti-ferromagnetic layer is arranged adjacent to the magnetization fixed layer. The magnetization free layer, the insulating layer, and the magnetization fixed layer are laminated in this order, and the MTJ (Magnetic Tunnel Junction) is formed therein. The magnetization fixed layer is formed of relatively hard ferromagnetic material, and the magnetization direction thereof is substantially fixed to one direction by the anti-ferromagnetic layer arranged in adjacent to it. The magnetization fixed layer functions as a reference in the operation of reading out. On the other hand, the magnetization free layer is formed of relatively soft ferromagnetic material. And a magnetic anisotropy is added to the magnetization free layer such that the magnetization direction thereof is to be either of a state in which it is parallel to the magnetization of the magnetization fixed layer or a state in which it is antiparallel to the magnetization of the magnetization fixed layer. The magnetization free layer functions as an information storage portion. The insulating layer is formed of insulating material. In the magnetic random access memory, information of “0” or “1” is stored correspondingly to whether the magnetization of the magnetization free layer is parallel to or antiparallel to the magnetization of the magnetization fixed layer.
For reading out information from the magnetic random access memory, the magnetoresistive effect is used. Namely, information is read out by detecting the difference of the resistance value of the MTJ caused by the difference of the relative angle between the magnetization of the magnetization free layer and the magnetization fixed layer by flowing an electric current through the MTJ.
On the other hand, various methods are proposed as the information writing method of the magnetic random access memory. The methods are grossly classified into the magnetic field writing method and the spin injection writing method.
The spin injection writing method is a method of inverting the magnetization of the magnetization free layer caused by the exchange of the spin torque with the magnetization fixed layer by changing the direction of current flowing through the MTJ. In the spin injection writing method, the current required for writing is proportional to the area of the MTJ. Therefore, the less the area of the MTJ is, the less the current value required for writing becomes. Namely, the spin injection writing method has an advantage in scaling property, and is expected as means to achieve a large capacity magnetic random access memory.
However, in the spin injection writing method, because relatively large current flows through the MTJ in writing, it is anxious that the writing tolerance may be weak. Further, a problem of the tolerance of the insulating layer is also pointed out as a problem for application. Moreover, for the spin injection magnetization inversion, longer time is required relatively to the magnetization inversion by the magnetic field so that it is disadvantageous for high-speed operation. Namely, there are many problems which bring out suspicions to the practical realization of a random access memory adopting the spin-injection writing method with high-speed and high-reliability.
On the other hand, the magnetization inversion by a magnetic field occurs by a time less than a nanosecond. Further, a large current does not flow through the insulating layer, so that the high-reliability is guaranteed. Therefore, the magnetic field writing method is preferred to the magnetic random access memory which can operate at high-speed.
The magnetic field writing method of the magnetic random access memory is realized generally by using the magnetic field being induced when a current flows in a writing interconnection arranged near the MTJ. And in most of the magnetic random access memories of the magnetic field writing method currently under researches and developments, each magnetoresistive effect element is arranged at a cross point of two writing interconnections orthogonally crossed with each other, and the writing is performed by the synthetic magnetic field applied to the magnetization free layer when currents flow in the two writing interconnections (in the followings, this is called as the two-axis writing method).
The most common two axis writing method is the asteroid method. In this method, currents are simultaneously flow in two orthogonal writing interconnections, and the magnetization of the magnetization free layer is inverted by their synthetic magnetic field. At this timing, the magnetic field induced by one writing interconnection is applied to the cells arranged in the same row or the same column with the selected cell, and these cells become a half-selected state. For avoiding the magnetization inversion under the half-selected state, the recording is required to be performed within a limited margin. Namely, the asteroid method has a problem of cell selectivity.
For solving the cell selectivity problem of the two axis writing method, the toggle method is proposed in Patent Literature 1 (U.S. Pat. No. 6,545,906). In the toggle method, currents flow in two orthogonal writing interconnections sequentially to invert the magnetization. In the toggle method, the problem of cell selectivity is almost perfectly solved. However, it is required to perform reading out before writing so that it is not suitable for high-speed operation.
On the other hand, in Patent Literature 2 (Japanese Patent Application Publication JP2004-348934A: U.S. family: U.S. Pat. No. 7,184,301(B2)), one axis magnetic field writing method is proposed. In this method, the above problems regarding selectivity and high-speed property are simultaneously solved. In this one axis magnetic field method, one cell has one writing interconnection, and the writing interconnection is connected to the source/drain of a MOS transistor. The gate of this MOS transistor is connected to a word line arranged along a first direction, and another source/drain of this MOS transistor is connected to a bit line arranged along a second direction. By such a structure, the problems regarding the cell selectivity and the high-speed property are simultaneously solved. So the one axis magnetic field method is a preferred method to achieve a magnetic random access memory which can operate at high-speed. In Non-Patent Literature 1 (J. Appl. Phys. 105, 07C921 (2009)) and Non-Patent Literature 2 (J. Appl. Phys. 103, 07A711 (2008)), the operation of the cell adopting the one axis writing method with the write current of 1 mA or less is confirmed.
For the high-speed operation of a memory, reading out at high speed is also required together with writing at high speed. For reading out at high speed, the difference between the signal levels of the resistance values of the element at “0” state and “1” state is required to be adequately large. Namely, it is required for the MR ratio to be adequately high. In Non-Patent Literature 3 (IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 42, NO. 4, APRIL 2007), a simulation result is represented in which the MR ratio of 70% or more is required for the reading out operation at high speed of 500 MHz or higher.
In recent years, a TMR film using an MTJ composed of CoFeB/MgO/CoFeB has been developed, and the MR ratio of over 200% has been reported. However, in the CoFeB film, the anisotropy of the material like the magneto crystalline anisotropy is large, and when it is used for a magnetization free layer, there is a problem that the value of the inversion magnetic field becomes large, and its application to the MRAM of the magnetic field writing type is difficult because of the largeness of the write current. On the other hand, in the NiFe magnetization free layer which is used for a common magnetic field writing cell as a magnetization free layer, though the write current is small, there is a problem that the MR ratio is also small.
As a related technique, in Patent Literature 3 (Japanese Patent Application Publication JP2003-198003A, U.S. family: U.S. Pat. No. 7,173,300(B2)), a magnetoresistive effect element, a manufacturing method thereof, and a magnetic memory device are disclosed. This magnetoresistive effect element has a free layer whose magnetization direction is invertible, and information is recorded by using the change of the magnetization direction of the free layer. In this magnetoresistive effect element, its free layer has a lamination structure including at least two layers. In the lamination structure, at least one layer is a ferromagnetic body layer composed of a ferromagnetic body, and at least one other layer is a low saturation magnetization ferromagnetic body layer whose saturation magnetization is smaller than the ferromagnetic body layer.
In Patent Literature 4 (Japanese Patent Application Publication JP2004-200245A, U.S. family: U.S. Pat. No. 7,379,280 (B2)), a magnetoresistive element and a manufacturing method of the magnetoresistive element are disclosed. This magnetoresistive element includes: an anti-ferromagnetic layer; a fixed ferromagnetic layer formed in junction with the anti-ferromagnetic layer and having a fixed spontaneous magnetization; a tunnel insulating layer formed in junction with the fixed ferromagnetic layer and being non-magnetic; and a free ferromagnetic layer formed in junction with the tunnel insulating layer and having an invertible free spontaneous magnetization. The fixed ferromagnetic layer has a first composite magnetization layer having a function of preventing at least one kind of material constituting the anti-ferromagnetic layer from being diffused to the tunnel insulating layer.
In Patent Literature 5 (Japanese Patent Application Publication JP2006-318983A), a magnetic memory element and a memory are disclosed. This magnetic memory element has a recording layer which retains information by the magnetization state of a ferromagnetic layer; a tunnel insulating film; and a magnetization fixed layer whose magnetization direction is fixed. A ferromagnetic layer constituting at least one of the recording layer and the magnetization fixed layer is composed of a compound which includes at least: one or more element selected from Fe, Co, and Ni; and B element, and has a crystalline structure. The period x of the unit crystalline structure of the crystal of the compound is in the range of: 0.19 nm≦x≦0.23 nm.
In Patent Literature 6 (Japanese Patent Application Publication JP2008-103728A, U.S. family: US2008088986(A1)), a magnetic tunnel junction element and a manufacturing method thereof are disclosed. This manufacturing method of the magnetic tunnel junction element is a manufacturing method of a magnetic tunnel junction (MTJ) element including a free layer having a magnetic moment. The method includes: a step of forming a magnetic pinning layer on a substrate; a step of forming a magnetic pinned layer on the magnetic pinning layer; a step of forming a tunnel barrier layer on the magnetic pinned layer; a step of forming a free layer composed of NiFe (nickel iron) and a first cap layer composed of NiFeHf (nickel iron hafnium) on the tunnel barrier layer; a step of forming a second cap layer composed by laminating Ta (tantalum) and Ru (ruthenium) in turn on the first cap layer; and a step of sharpening an edge surface of the tunnel barrier layer and the free layer by performing heating treatment under a condition of time and temperature enough to diffuse oxygen captured in the free layer into the first cap layer.
In Patent Literature 7 (Japanese Patent Application Publication JP2009-117846A, U.S. family: US2009122450(A1)), a TMR element and a forming method thereof are disclosed. This TMR (tunneling magnetoresistive) element includes: a laminated body having a seed layer, anti-ferromagnetic layer, and a pinned layer laminated on a substrate in turn; a tunnel barrier layer formed on the pinned layer and composed of MgOx (magnesium oxide); a free layer formed on the tunnel barrier layer and including CoBX (cobalt boron; 1 atom %≦X≦30 atom %) or FeBV (iron boron; 1 atom %≦V≦30 atom %); and a cap layer formed on the free layer.
In Patent Literature 8 (Japanese Patent No. 3888463, U.S. family: U.S. Pat. No. 7,184,301(B2)), a memory cell and a magnetic random access memory are disclosed. This memory cell includes: a first transistor having a first gate, a first terminal serving as a terminal of one side other than the first gate, and a second terminal serves as a terminal of another side; and a magnetoresistive element which has a third terminal having a spontaneous magnetization whose magnetization direction is inverted correspondingly to the recorded data and serves as a terminal of one side, and a fourth terminal serves as another terminal. The first terminal is connected to a first bit line. The second terminal is connected to a second bit line. The first gate is connected to a first word line. The third terminal is connected to a second word line. The fourth terminal is connected to the second terminal.